I'm getting mileage fairly equivalent to what some hybrid owners are getting. Granted, it's in one of the few cars that's even uglier than a Prius, but at least I don't have Prius stigma. And, the car was free-to-me.

Now we're talkin! Isn't Renault a French company?! Maybe the French government will get them to dump this project.
My very first car was a 1970 Opel Kadette wagon. Great car. 1.1 litre beater. Returned 30+MPG with a manual trans. 30+ US MPG in 1970. My father and I rebuilt that motor twice in 300K miles.

And when I bought my Subaru I considered the Jetta TDI wagon but I couldn't get past the poor reliability record of the newest versions. Loved the fuel efficiency, though.

The Volt does connect the ICE directly to wheels at times; mostly at highway speeds.

In total ownership cost, a hybrid should match or beat its ICE only equivalent, but there are reasons beyond fuel costs that people buy them. The UK, and perhaps other countries, the pricing can be different. An UK user of Priuschat regularly complains about how Toyota overprices the Prius there.

It is possible to match a hybrid with some cars in fuel efficiency, but most people aren't willing to drive for efficiency to do so, and the hybrid is likely larger and more comfortable. Most hybrids driven for efficiency will squash an efficiently driven driver can do. A diesel can beat them, but not when stop and go city routes are a factor.

Another thing to remember with diesels is that there is no sacrifice of fuel if you want a "Spirited" drive. Top gear managed to get the Prius down to 19 MPG when driving quick. I took my car for a Spirited drive the other night, reset the trip meter to see what kind of economy I got. Took the revs to 3k/4k (high for a diesel) dropped a cog and floored the throttle on every bend, accelerated hard throughout the journey. Result, exactly 50 MPG. No compromise.

Top Gear went a bit beyond 'spirited' when driving the Prius. Even Consumer Reports mid 30s mpg for city is puzzling to anybody that has driven the car for a bit. The worse I ever saw was 38mpg when doing high speed freeway(70+ mph). It might into the high 40s for mpg with frisky town driving.

Then for those that care for emissions beyond just carbon, it is tough to match a hybrid. While there might be some diesels in the US that could do so, the manufacturer chose to certify to a less clean bin.